1. Field of the Invention
This invention relates to a magnetic recording medium and a process for producing the same. More specifically, the invention relates to a coated magnetic recording medium that is capable of efficient electromagnetic conversion characteristics in a high-frequency range and which can be manufactured at high production rate. The invention also relates to a process for producing such magnetic recording medium.
2. Description of Related Art
With the recent spread of large-capacity storage devices, a demand has arisen for increasing the density of magnetic recording media and to meet this need, more highly filled-up, thinner and smoother magnetic layers are required. However, the efforts to increase the filling up of the magnetic layer and smoothen its surface are compromised by the deterioration of the physical properties of the magnetic recording medium as the final product. A common practice in dealing with this problem is preventing it by providing a separate non-magnetic layer under the magnetic layer.
While several methods have been proposed to date, Unexamined Published Japanese Patent Application (kokai) No. 207027/1984 teaches the use of a radiation-cured undercoat containing a radiation-curable resin and a filler with a view to preventing the sticking of adjacent layers while increasing the filling-up and wear resistance of the magnetic layer and improving the surface property of the resulting medium. However, the proposed undercoat is not satisfactory in terms of dispersibility and surface property and, in addition, the physical properties of the undercoat and the overlying magnetic layer do not have satisfactorily high reliability in varying environments ranging from high to low temperatures.
Unexamined Published Japanese Patent Application (kokai) No. 167225/1992 teaches the technique of increasing the stiffness and improving the electromagnetic conversion characteristics by incorporating acicular particles with an aspect ratio of 3 or more in a binder resin in the undercoat that cures upon irradiation with electromagnetic waves such as radiations and UV rays. However, the proposed undercoat is still unsatisfactory in terms of dispersibility and surface property.
Other relevant prior art references include Unexamined Published Japanese Patent Application (kokai) Nos. 38723/1985 and 131652/1994. The former teaches the technique of producing a magnetic recording medium with small static charge buildup, high surface smoothness of the magnetic layer and high adhesive strength by using a radiation-cured undercoat that has carbon black dispersed in one or more radiation-curable oligomers or polymers that contain at least one unsaturated double bond. According to the latter reference, two electron beam (EB) curable resins having polar groups are used in a back coat. The combined use of two EB curable resins having polar groups is intended to improve the dispersibility of inorganic pigments such as carbon so that print-through to the magnetic layer is sufficient retarded to achieve better electromagnetic conversion characteristics and lower error rates. However, this reference makes no disclosure about the application of the proposed technique to the non-magnetic undercoat.
Thus, various methods have so far been proposed with a view to improving the characteristics of magnetic recording media by providing an undercoat and employing EB curable resins. However, there still remains the need to develop techniques capable of producing even better magnetic recording media in order to satisfy the recent requirement for higher density.